The vertebral spine is the axis of the skeleton on which all of the body parts “hang.” In humans, the normal spine has seven cervical, twelve thoracic, and five lumbar segments. The lumbar spine sits upon the sacrum, which then attaches to the pelvis, and in turn is supported by the hip and leg bones. The bony vertebrae of the spine are separated from one another by intervertebral discs which, as described below, act as a complex joint and provide a compressive load bearing structure.
With reference to FIG. 1, the typical vertebra 10 (such as the lumbar vertebra shown) has a thick anterior bone mass called the vertebral body 12. A vertebral or neural arch 14 is posteriorly defined relative to the vertebral body 12 via opposing pedicles 16. Laminae 18 are formed at the posterior side of the pedicles 16 and combine to form a spinous process 20. Thus, the spinous process 20 projects from the posterior region of the vertebral arch 14. In addition, transverse processes 22 project laterally from the respective pedicle 16/lamina 18 interface. Similarly, an opposing pair of superior articular processes 24 project upwardly from the respective pedicle 16/lamina 18 junction, each terminating in medially upward-facing facet 26. Conversely, two inferior articular processes 28 project downwardly from the respective pedicle 16/lamina 18 junction, and also terminate in a facet 30 (best shown in FIG. 2) that otherwise face laterally downward. As described below, the facets 26 or 30 interface with corresponding facets 26 or 30 of an adjacent vertebra to form part of a joint complex.
The center of adjacent vertebral bodies 12 are supported by an intervertebral disc 34. The intervertebral disc 34 primarily serves as a mechanical cushion permitting controlled motion within or between vertebral segments of the axial skeleton. The normal disc 34 is a unique, mixed structure, comprised of three component tissues: a nucleus pulposus (“nucleus”) 36, an annulus fibrosus (“annulus”) 38, and opposing vertebral end plates (not shown). The vertebral end plates are composed of thin cartilage overlying a thin layer of hard, cortical bone which attaches to the spongy, richly-vascular, cancellous bone of the corresponding vertebral body 12. The end plates thus serve to attach adjacent vertebral bodies 12 to the disc 34. The annulus 38 is a tough, outer fibrous ring consisting of 15 to 20 overlapping, multiple plies. Immersed within the annulus 38 is the nucleus 36. The healthy nucleus is largely a gel-like substance having a high water content, and like air in a tire, serves to keep the annulus 38 tight yet flexible. The nucleus 36 moves slightly within the annulus 38 when force is exerted on the adjacent vertebrae 10 while bending, lifting, etc.
With the above in mind, physiology of the spinal column is oftentimes described in terms of a functional spinal unit 50 as shown in FIG. 2. The functional spinal unit 50 consists of opposing, superior and inferior vertebrae 10a, 10b, and the intervening disc 34 that combine to generally define an anterior region or column 52 and a posterior region or column 54. The anterior region 52 consists of the opposing vertebral bodies 12a, 12b and the disc 34. Conversely, the posterior region 54 consists of the posterior portions of the vertebrae 10a, 10b (i.e., posterior of the respective vertebral bodies 12a, 12b), and the points of interface therebetween. In this regard, the inferior articular processes 28a (one of which is shown in FIG. 2) of the superior vertebra 10a engage with corresponding ones of the superior articular processes 24b (one of which is shown in FIG. 2) of the inferior vertebra 10b. The facets 30 (best shown for the inferior vertebra 10b in FIG. 2) of the superior vertebra's 10a inferior articular processes 28a nest or engage with a corresponding one of the facets 26 (FIG. 1) of the inferior vertebra's 10b superior articular processes 24b. When the adjacent vertebrae 10a, 10b are aligned, the corresponding, mated facets 26, 30 are encapsulated within cartilage and ligaments, forming an interlocking facet joint 56 (one of which is visible and referenced generally in FIG. 2). This is commonly referred to as a zygoapophyseal joint. In light of the vertebral anatomy, then, the posterior region 54 is characterized as including two of the facet joints 56 that, along with corresponding ligament structures (e.g., ligamentum flavum, intraspinous ligament, and supraspinous ligament), can collectively be referred to as the posterior joint complex. In addition, the disc 34 serves as a joint at the anterior region 52 such that the functional spinal unit 50 can be considered as establishing a three-joint complex including an anterior joint or anterior joint complex (i.e., the disc 34) and a posterior joint complex.
The physiological functions of these three joints are intimately linked. In general terms, the anterior joint complex 34 provides the primary compressive load bearing structure (i.e. axial compliance) and assists with rotational stability for the functional spinal unit 50, whereas the posterior joint complex provides primary motion (i.e., flexion, extension, and rotation) control. However, each joint of the three-joint complex affects these biomechanical functions. That is to say, the posterior joint complex 56 assists (or at least does not overtly impede) in supporting the functional spinal unit 50 when subjected to an axial or compressive load; similarly, the anterior joint complex 34 assists (or at least does not overtly impede) the posterior joint complex 56 in controlling motions of the functional spinal unit 50. Along these same lines, damage to one joint may lead or cascade to impairment of the opposing joint complex.
Painful, disabling degeneration of the functional spinal unit 50 can result from a number of different spinal pathologies that may increase in severity over time. In most instances, however, the initial degeneration of the functional spinal unit 50 is focused upon either the anterior region 52/anterior joint complex 34 or the posterior region 54/posterior joint complex 56. For example, disruption of the anterior joint, through disease or injury, can be attendant by a bulging or tearing of the annulus and/or nucleus degeneration. The resulting discal degeneration and/or loss of disc height can contribute to persistent and disabling back pain. Similarly, through disease or trauma, the ligamentous structures, laminae, spinous process, articular processes, and/or facets can become damaged (e.g., synovitas, subluxation of facet joints, osteophyte formation, etc.), resulting in an undesired anatomy, loss and/or change of mobility, and pain or discomfort.
In light of the above, treatment of a patient suffering from back pain or other spinal-related malady initially entails the physician identifying the location and form of primary degeneration (i.e., the anterior region 52/anterior joint complex 34 or posterior region 54/posterior joint complex 56). Once the joint pathology has been diagnosed, an appropriate treatment is selected. In some instances, the only viable treatment is complete fusion of both the anterior region 52 and the posterior region 54 of the functional spinal unit 50. Total (or 360°) fusion prevents any motion of the functional spinal unit 50 from occurring and thus is employed only in the most severe cases. More preferably, the selected treatment maintains or permits as much mobility of the functional spinal unit 50 as possible.
In recognition of the above, a plethora of non-fusion, motion preservation, prostheses, stabilization systems, etc., have been developed to correct degenerative pathology of either the anterior region 52/anterior joint complex 34 or the posterior region 54/posterior joint complex 56. For example, total disc replacement devices designed to replace the entire disc (nucleus and annulus) and restore motion are available from DePuy and Synthes Spine, to name but two. Also, prosthetic intervertebral disc nucleus devices by Raymedica, Disc Dynamics, and others focus upon replacing just the nucleus and mimic the columnar support provided by the natural disc. Conversely, various posterior joint repair systems and devices have been developed. For example, a variety of posterior stabilization technologies are available for controlling and/or restoring motion, such as spinal facet joint prosthesis from Archus Orthopedics, Inc. and Facet Solutions, Inc.; spinous process devices from Abbott Spine, Medtronic Sofamor Danek, and Paradigm Spine, LLC; and pedicular-based systems from Zimmer Spine and Applied Spine Technologies, Inc.; to name but a few.
It has surprisingly been discovered that while the various spinal treatment devices and methods may provide significant initial improvements to the particular joint complex being repaired, longer term implications on the opposing joint complex, and thus the functional spinal unit 50 as a whole, are not taken into consideration and thus are not addressed with the current technologies mentioned above. As a result, any “repair” directed toward one of the anterior or posterior joints may actually lead to or cause degeneration of the opposing joint complex due to the intimately linked nature of the functional spinal unit's entire three joint complex.
In light of the above, a substantial need exists for systems and methods for stabilizing a functional spinal unit in a manner facilitating proper biomechanical functioning in load sharing and motion control of the three-joint complex, as well as methods for selecting appropriate system components.